Columnar-grained nickel-base superalloy castings

ABSTRACT

The mechanical properties of columnar-grained nickel-base superalloy castings are improved by the addition of 1.5-3.5 weight percent hafnium to the alloy composition.

Field of Search ..75/17i, 170; 148/32, 32.5

= il v Eates tent 1 1 .1 3,711,337 Sullivan et a1. 1 Jan, 16, 1973 54 QOLUMNAR-GRAINED NICKEL-BASE [56] References Cited- SUPE "LLQ CASTINGS p y UNITED STATES PATENTS [75] Inventors: Cornelius P. 'Suliivan',* Branford; 3260 505 7/1966 d 1 5/ 7 v er ny er F v Duh! Newmgtfm both'of 3,005,705 10/1961 Cochardt... ....75 171 3,526,499 9 1970 Quigg at al. [73] Assignee: United Aircraft Corporation, East Hartford, Conn Primary E xaminerRichard 0. Dean 1 3 AttorneyRichardN. James [22] Filed:' Dec. 16, 1970 w p 21 Appl.- No.: 98,794 ABSTRACT 4 I The mechanical properties of columnar-grained nickel-base superalloy castings are improved by the [2? ..148/3.25i7i/91/(71) addition of 1.5 35 weight percemvhafnium to the n c alloy composition I 2 Claims, 3 Drawing Figures PARNIEBJM man 3. 71 1, 3 37 saw 1 or z A TJE/Vf/ COLUMNAR-GRAINED NICKEL-BASE SUPERALLOY CASTINGS BACKGROUND OF THE INVENTION The present invention relates to columnar-grained nickel-base superalloy castings.

The common mode of failure of the high temperature nickel-base alloys in creep involves cracking along grain boundaries essentially normal to the stress axis. Previous workers have demonstrated that minor additions of elements such as born, cerium, magnesium and zirconium significantly improve the creep properties of these alloys. Thus, these additions have been widely used although the detailed mechanisms responsible for the improved properties attributable to them have not been fully resolved at this time, although it is generally believed that some form of grain boundary interaction is involved. In a similar vein, Cochardt U.S. Pat. No.

3,005,705 and Belgian patent 732,244 have reported that the addition of larger amounts of hafnium enhances creep life by increasing ductility of polycrystalline cast alloys.

ture grain boundary fracture has been disclosed by Ver Snyder U. S. Pat. No. 3,260,505. In this development those grain boundaries oriented transverse to the major: stress axis are essentially eliminated by directional solidification techniques which produce castings wherein the grain boundaries are aligned along the stress axiscolumnar-grained castings. In the case of the very strong alloys, such as Alloy A having a nominal composition in weight percent of 0.15 C, 9.0 Cr, 10.0 Co, 5.0 A1, 2.0 Ti, 12.5 W, 1.0 Cb, 0.015 B, 0.05 Zr,

balance Ni, this technique has proved superior to those relying solely upon alloy chemistry modifications to improve high temperature grain boundary properties. However, when stressed across the grain boundaries, the columnar-grained material retains its polycrystal line characteristics.

FIG. 2'1? aphotomicrograph ofthe in icrostructure of DESCRIPTION OF THE PREFERRED EMBODIMENT The most striking effect of a nominal 2 weight percent hafnium addition to columnar-grained Alloy A is the enhancement of transverse creep ductility and lifetime. These results, summarized in Tables I and II are for 0.1 inch thick flats in a fully heat treated condition (2,200F./2 hours plus l,975F./4 hours plus 1,600F./hours) and are compared to the results for the same alloy with no hafnium. It should be noted that the stress levels utilized were quite high to accelerate testing.

TABLE 1 EFFECT OF I-IAFNIUM ON TRAN SVERSE CREEP PROPERTIES OF COLUMNAR-GRAINED f.o.l. Failed on Loading TABLE II [Efiect o1 hafnium on the tensile properties of columnar-grained casting] Temperature F.) I.'.T.S. (K s.i.) 0.2% Y.S. (K s.i.) Elongation (percent) R. 01A. (percent) 2 w/o Hf N0 Hi 2 w/o HI No Hi 2 w/o Hi No HI 2 wlo Hi No Hi R 133 118 117 116 9. 0 3. 0 l4. 5 6. 2 141 123 117 7. 2 1. 7 13. 5 1. 9 B 85 76 67 57 17. 4 4. 2 15. 3 2. 4 161 153 136 137 6. 1 7. 7 8. 3 8. 6 172 172 140 9. 0 7. 5 14. 0 8. 6 89 85 69 65 21. 8 18. 0 28. 0 25. 6

The columnar grains in the transverse specimens are perpendicular to the tensile axis, whereas they are parallel to the tensile axes in the longitudinal specimens. This results in a higher resolved shear stress on the slip planes (Schmid (actor) for the transverse specimens, thereby lowering the measured yield strength.

SUMMARY OF THE INVENTION The present invention contemplates the addition of 1.5-3.5 weight percent hafnium to the columnar- 60 grained nickel-base superalloys.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a photomicrograph of the microstructure of -a columnar-grained nickel-base superalloy casting without hafnium after 2,200F./2 hours plus 1,975F./4

. hours plus 1,600F./ 32 hours.

hafnium contents. However, there appears to be a critical range of hafnium outside of which no beneficial effects are observed, as seen in FIG. 3. Below about 1.5 weight percent and above about 3.5 weight percent hafnium, the creep ductilities fall to the low values found in the hafnium-free material.

In addition to impartin'g additional ductility to the transverse properties of the casting at l,400F., hafniurn alters the fracture characteristics. When hafnium is absent both creep and tensile fractures are entirely intergranular. The addition of hafnium results in a fracture with extensive areas of crystallographic cracking. The crystallographic mode of crack propagation indicates enhanced cracking resistance of the grain boundaries. This'transgranular fracture made at 1,400F. appears to be a characteristic of the hafnium effect in columnar-grained castings.

The addition of hafnium alters the microstructure of columnar-grained Alloy A as seen in FIGS. 1 and 2.

Additions of hafnium increase the amount of the eutectic ('y/y) constituent normally present in this alloy. The carbides are also affected. instead of the single (Ti, Cb, W) C carbide normally found, two MC carbides of different lattice parameter are formed. A carbide of the larger lattice parameter (a 4.59 A) suggests that it is hafnium-rich, with some dilution by elements of smaller atomic size tending to decrease a, below the value for HfC (a =4.64 A). The other MC carbide has a lattice parameter slightly larger than the MC type usually found in this alloy (4.40 vs. 4.36 A) which suggests that some hafnium is present in this carbide causing a lattice parameter increase. X-ray diffraction phase identification of electrolytically extracted residues has indicated that M C carbides are only present after heat treatment. This is in contrast to the hafnium-free alloy where some of these carbides are present in the as-cast condition. Contrary to the results of others working with the polycrystalline castings, the results indicate that hafnium produces no obvious refinement in MC carbide size.

In summary, the addition of a critical amount of hafnium to the columnar-grained nickel-base superalloy castings results in a significant improvement in the transverse creep properties while maintaining the excellent longitudinal mechanical properties associated with the columnar-grained structure. Although the invention has been described in detail in connection with certain preferred embodiments, these are illustrative only for obvious modifications will occur to those skilled in the art within the true spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. A casting consisting essentially of an alloyof the nominal chemistry, by weight, 0.15 percent carbon, 9 percent chromium, l0 percent cobalt, 5 percent aluminum, 2 percent titanium, 12.5 percent tungsten, 1 percent columbium, 0.015 percent boron, 0.05 percent zirconium, 1.5-3.5 percent hafnium, balance nickel, having a columnar-grained structure.

2. A casting according to claim 1 wherein the hafnium content is about 2 percent by weight. 

2. A casting according to claim 1 wherein the hafnium content is about 2 percent by weight. 